Data transfer apparatus, network system, and data transfer method

ABSTRACT

Whether the node to which data has been sent is connected to a bus is determined according to destination information, and, when it is determined that the node is not connected to the bus, predetermined error information is sent to the data transmission source. Therefore, data re-transmission is prevented and thereby the frequency band of a network is efficiently used.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to data transfer apparatuses, networksystems, and data transfer methods, and more particularly, to thosesuited to a digital-serial-data interface apparatus which performsarbitration for a bus use right prior to data transfer.

2. Description of the Related Art

There has been known the IEEE-1394 high-performance serial busspecification (hereinafter called the IEEE-1394 specification) as aninterface specification which supports high-speed data transfer andreal-time transfer and which is to be used for an interface formultimedia-data transfer.

In this IEEE-1394 specification, data transfer speeds of 100 Mbps(98.304 Mbps), 200 Mbps (196.608 Mbps), and 400 Mbps (393.216 Mbps) aredefined, and a 1394 port having a higher transfer speed can be used at alower transfer speed. With this feature, the data transfer speeds of 100Mbps, 200 Mbps, and 400 Mbps can be used at the same time in anidentical network.

The IEEE-1394 specification employs a transfer format conforming to adata/strobe link (DS-Link) encoding method, in which transfer data isconverted to two signals, a data signal and a strobe signal forcompensating for the data signal, and a clock is generated by applyingan exclusive OR operation to the two signals, as shown in FIG. 1.

In addition, a cable 1 is specified, which has a structure in which twosets of twisted pairs (signal lines) 3 each shielded by a first shieldlayer 2 and power lines 4 are bundled and then shielded by a secondshield layer 5, as shown by a cable structure illustrated in a sectionalview of FIG. 2.

In the IEEE-1394 specification, two types of connection methods, a daisychain and a node branch, can be used. In the daisy chain, up to 16 nodes(units having a 1394 port) can be connected and the maximum lengthbetween nodes is 4.5 m. As shown in FIG. 3, with the node branch beingused together, up to 63 nodes (physical node addresses) can beconnected.

In the IEEE-1394 specification, a cable having the above structure canbe connected and disconnected while a unit is operating, namely, whilethe power is on. When a node is added or removed, a 1394 network isautomatically re-structured. In this case, connected node units areautomatically recognized and the IDs and arrangement of the connectedunits are managed by an interface.

FIG. 4 shows the components and protocol architecture of an interfaceconforming to the IEEE-1394 specification. The interface is divided intohardware and firmware.

The hardware is formed of a physical layer (PHY) and a link layer.

The physical layer directly drives signals conforming to the IEEE-1394specification. The link layer is provided with a host interface and aninterface with the physical layer.

The firmware includes a transaction layer formed of a management driverwhich applies actual operations to the interface conforming to theIEEE-1394 specification, and a management layer formed of anetwork-management driver conforming to the IEEE-1394 specification,called serial bus management (SBM).

An application layer includes software used by the user, and managementsoftware which interfaces the transaction layer and the managementlayer.

In the IEEE-1394 specification, transfer operations performed in anetwork are called sub-actions, and the following two types ofsub-actions are specified. As the two sub-actions, asynchronous transfermode called “asynchronous,” and synchronous transfer mode which assuresa transfer frequency band, called “isochronous,” are defined.

Each sub-action is further divided into three parts. They are transferstates called “arbitration,” “packet transmission,” and“acknowledgement.” In the “isochronous” mode, the “acknowledgement”state is omitted.

In an asynchronous sub-action, asynchronous transfer is performed. InFIG. 5, which indicates transition states along the time axis in thetransfer mode, a first sub-action gap indicates that a bus is idling. Bymonitoring the period of a sub-action gap, it is determined whether theprevious transfer is finished and new transfer is possible.

When an idling state continues for more than a predetermined period, anode which wants to transfer data determines that the bus is available,and performs arbitration in order to obtain the bus control right. Asshown in FIG. 6A and FIG. 6B, a node A serving as a root actuallydetermines whether the bus is stopped.

Next, a node which has obtained the bus control right in arbitrationexecutes data transfer, namely, packet transmission. After the datatransfer, a node which has received the data sends back an ACK (ACK:receiving-confirmation return code) corresponding to the result ofreceiving of the transferred data to execute acknowledgement. With theexecution of this application, both the transmission node and thereceiving node can check by the contents of the ACK that the transferwas successfully performed.

Then, the state returns again to a sub-action gap, namely, a bus idlingstate, and the above-described transfer operations are repeated.

An isochronous sub-action basically performs transfer having the samestructure as asynchronous transfer. As shown in FIG. 7, a higherpriority is given to the execution of isochronous transfer than that ofasynchronous transfer in an asynchronous sub-action.

Isochronous transfer in an isochronous sub-action follows a cycle-startpacket issued by the root node at a frequency of about 8 kHz, and has apriority over asynchronous transfer in an asynchronous sub-action.Therefore, the mode is the transfer mode which assures a transferfrequency band, and real-time data transfer is implemented.

When a plurality of nodes perform isochronous transfer of real-time dataat the same time, a channel ID for differentiating a content(transmitting node) is assigned to transfer data and only the requiredreal-time data is received.

FIG. 8 shows the structure of an address space in the IEEE-1394specification. The structure conforms to a CSR architecture (hereinaftercalled a CSR architecture) having 64-bit fixed addressing, defined inthe ISO/IEC 13213 specification.

As shown in FIG. 8, higher 16 bits indicate a node ID in each address togive an address space to a node. The node ID is divided into a 10-bitbus number and a six-bit node number, and higher 10 bits indicate a busID and lower six bits indicate a physical ID. Since either field usesthe value formed of all “1” bits for a special purpose, this addressingmethod gives 1023 buses and 63 separately addressable nodes for eachbus.

In the above-described IEEE-1394 specification, however, variousrestrictions apply in terms of a scale and ease of handling, such asthose in the number of connected units, a hop count, and a transferfrequency band. To relieve these restrictions and to expand a networkscale, a 1394 bus bridge specification has been examined.

In a status control register employed in the IEEE-1394 specification, a10-bit bus number field and a six-bit node number field are defined, andone bus is formed according to the node number field.

Among these fields, the node number field indicates 63 nodes in one busand their behaviors are specified in the IEEE-1394 specification. Incontrast, with the use of the 10-bit bus number field, when numbers areassigned to this field, up to 1032 buses are generated. A protocol forthe entire 1394 network will be specified in the 1394 bus bridgespecification.

A 1394 bridge has a function for propagating data over buses and needsto be disposed between buses. The 1394 bridge is formed of one set oftwo nodes called portals. Each portal performs processing for both thebus to which the portal is connected and the bus to which the otherportal is connected.

FIG. 9 shows a 1394 network using such a 1394 bridge. A circle connectedbetween 1394 buses indicate a 1394 bridge, and each semi-circleindicates a portal. In addition, as shown in FIG. 10, up to 1023 busescan be connected by using connections between buses with the use of 1394bridges.

Two standardization operations have been currently performed for the1394 bus bridge. One is that performed by an IEEE-1394.1 working groupand its contents are disclosed as a draft. The other is that performedby a broadband radio access network (BRAN) project under EuropeanTelecommunication Standard Institute (ETSI) in Europe, and its contentsare also disclosed as a draft.

The difference between them is that the IEEE-1394.1 working group hasbeen examining the 1394 bus bridge for general use, in which up to 1023buses can be used and the maximum hop count is 1022 whereas the BRANproject has been examining for home use with simplified functions,thereby generating a slight restriction on a topology structure.

A 1394 network of the BRAN specification supports up to 64 buses and upto two hops, and has a structure in which a number of buses (leaf buses)are connected around a central bus (branch bus) as shown in FIG. 11.

Bus-ID assignment is also simplified. As shown in FIG. 11, virtual IDsof 0 to 62 are assigned to portals connected to the branch bus. Thevirtual IDs are also used as the bus IDs of the leaf buses connected tothe other-side portals, and a bus ID of 63 is assigned to the branchbus. Bus-ID assignment is automatically performed in this way.

The above-described BRAN specification for the 1394 bridge issimplified, and has an advantage that packet transfer can be performedwithout referring to a routing table in asynchronous packet transferperformed over a bridge.

Such a case will be specifically described by referring to FIG. 12. In a1394 network having a structure like that shown in FIG. 12, it isassumed that asynchronous packet transfer is performed in a directionindicated by an arrow A and in a direction indicated by an arrow B. Inpacket transfer in the direction indicated by the arrow A, when thetransmission destination bus ID of an asynchronous packet is not the ownbus ID, namely, the transmission destination is other than the bus ID 2,the packet is forwarded in the direction indicated by the arrow A.

In packet transfer in the direction indicated by the arrow B, when thetransmission destination bus ID of an asynchronous packet is equal tothe bus ID of the bus connected there, namely, only when the packet is apacket bound for the bus ID 2, the packet is transmitted in thedirection indicated by the arrow B.

As described above, asynchronous packet transfer is achieved over abridge without using a routing table. However, what kind of processingis applied to a packet having the bus ID indicated by the transmissiondestination address, which does not exist on the current 1394 network,is not defined.

Therefore, when an asynchronous packet is sent to a destination which isnot on the network, it is not reported to the transmission source thatthe packet is an error packet and acknowledgement is not returned. Thetransmission source may repeat meaningless re-transmission.

SUMMARY OF THE INVENTION

The present invention has been made in consideration of the above point.Accordingly, an object of the present invention is to provide a datatransfer apparatus, a network system, and a data transfer method which,if a packet having a transmission destination node which does not existis erroneously transmitted, regard the packet as an error packet andperform normal error processing.

The foregoing object is achieved in one aspect of the present inventionthrough the provision of a data transfer apparatus for connecting busesand for transmitting data transferred through its own bus among thebuses to another bus among the buses, if necessary, according todestination information attached to the data, including transmittingmeans for determining according to the destination information whetherthe node serving as the destination of the data is connected to one ofthe buses, and, when it determines that the node is not connected, fortransmitting predetermined error information to the data transmissionsource. Therefore, data re-transmission is prevented, and thereby thefrequency band of a network is efficiently used.

The foregoing object is achieved in another aspect of the presentinvention through the provision of a network system in which a pluralityof buses are connected through a data transfer apparatus for connectingbuses and for transmitting data transferred through its own bus amongthe buses to another bus among the buses, if necessary, according todestination information attached to the data, wherein the data transferapparatus includes transmitting means for determining according to thedestination information whether the node serving as the destination ofthe data is connected to one of the buses, and, when it determines thatthe node is not connected, for transmitting predetermined errorinformation to the data transmission source. Therefore, datare-transmission is prevented, and thereby the frequency band of anetwork is efficiently used.

The foregoing object is achieved in still another aspect of the presentinvention through the provision of a data transfer method for a datatransfer apparatus for connecting buses and for transmitting datatransferred through its own bus among the buses to another bus among thebuses, if necessary, according to destination information attached tothe data, the data transfer method including a first step of determiningaccording to the destination information whether the node serving as thedestination of the data is connected to one of the buses; and a secondstep of, when it is determined that the node is not connected,transmitting predetermined error information to the data transmissionsource. Therefore, data re-transmission is prevented, and thereby thefrequency band of a network is efficiently used.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a timing chart of signals in data transfer in the IEEE-1394specification.

FIG. 2 is a sectional view of a cable specified in the IEEE-1394specification.

FIG. 3 is an outlined view showing the structure of a network whichemploys the IEEE-1394 specification.

FIG. 4 is an outlined view showing the components and protocolarchitecture of an interface conforming to the IEEE-1394 specification.

FIG. 5 is an outlined view showing a packet in asynchronous transfer.

FIG. 6A and FIG. 6B are outlined views showing bus-use-right acquisitionstates caused by arbitration.

FIG. 7 is an outlined view showing packets in isochronous transfer.

FIG. 8 is an outlined view showing addressing in a CSR architecture.

FIG. 9 is an outlined view showing the basic structure of a 1394 networkusing a 1394 bridge.

FIG. 10 is an outlined view showing the structure of a 1394 networkusing a plurality of 1394 bridges.

FIG. 11 is an outlined view showing the structure of a 1394 network ofthe BRAN specification.

FIG. 12 is an outlined view showing routing in a 1394 network of theBRAN-specification.

FIG. 13 is an outlined view showing the structure of a 1394 network ofthe BRAN specification.

FIG. 14 is an outlined view showing packet processing in a portal on abranch bus.

FIG. 15 is an outlined view showing packet processing in a portal on aleaf bus.

FIG. 16 is an outlined view showing the structure of a 1394 network ofthe BRAN specification.

FIG. 17 is an outlined view showing the structure of a 1394 network ofthe BRAN specification.

FIG. 18A and FIG. 18B are outlined views showing bridge bits in a selfID packet.

FIG. 19 is an outlined view showing a correspondence table between nodeIDs and virtual IDs.

FIG. 20 is an outlined view showing a correspondence table between nodeIDs and virtual IDs.

FIG. 21 is an outlined view showing packet processing in a portal on abranch bus.

FIG. 22 is a block diagram showing the structure of an interfaceapparatus having a bridge function.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention will be described in detail byreferring to the drawings.

In a 1394 bridge of the BRAN specification, asynchronous transmissionover the bridge is allowed. A method for performing asynchronoustransfer over the bridge will be described first.

When a node existing on a bus connected to the 1394 bridge sends anasynchronous packet to a node existing on a bus other than the localbus, the bridge serves as a repeater point. A case like that shown inFIG. 13 will be considered.

A 1394 network 50 shown in the figure is formed of a bridge 53 formed ofa portal 51 connected to a bus BUS1 and a portal 52 connected to a busBUS63, a bridge 56 formed of a portal 54 connected to the bus BUS63 anda portal 55 connected to a bus BUS2, a node 57 connected to the busBUS1, and a node 58 connected to the bus BUS2.

In such a structure, the bus BUS63 connected to two portals or more iscalled a branch bus, and the bus BUS1 and the bus BUS2 each connected toonly one portal are called leaf buses.

When the node 57 performs packet transmission to the node 58, a packetsent from the node 57 is first received by the portal 51. Receiving thepacket, the portal 51 determines whether the packet is to be forwardedto an adjacent bus. In other words, the transmission destination bus IDof the packet is checked.

When the transmission destination bus ID is not the local-bus ID, thepacket is regarded as that bound for a node disposed on another bus, andis actually forwarded. To forward the packet, the portal 51 firstreturns an ACK indicating that the packet has been received, to the node57.

Then, the portal 51, which received the packet, passes the packet to theportal 52, positioned at the other side of the bridge 53, and the portal52 sends the received packet to the bus BUS63 connected thereto.

The packet, which was sent from the portal 52 to the bus BUS63, is thenreceived by the portal 54. Receiving the packet, the portal 54determines whether the received packet is to be forwarded to an adjacentbus, in the same way as the portal 51 did.

It is noted that this determination processing differs depending onwhether portal 54 is connected to a branch bus or to a leaf bus. Theprocessing is shown in FIG. 14 and FIG. 15.

Since the portal 54 is connected to a branch bus, whether the packet isto be forwarded means whether the bus ID of the transmission destinationaddress matches the adjacent bus ID.

In the present case, since the bus ID of the transmission destinationaddress is the bus BUS2, the portal 54 determines that the packet is tobe forwarded, and passes the packet to the portal 55, positioned at theother side. The portal 54 also returns an ACK indicting that the packethas been received, to the portal 52.

The portal 54, which received the packet, sends the packet to the busBUS2 connected thereto, and the node 58, disposed on the bus BUS2, canreceive the packet. Receiving the packet, the node 58 returns an ACK tothe portal 55 to report that the packet has been received. Through suchprocessing, asynchronous transmission is performed over bridges.

When the node 58 sends a packet to the node 57, the same processing asthat described above is performed through the above-described path inthe opposite direction to achieve isochronous packet transmission overbridges.

The method for performing asynchronous transmission over 1394 bridges ofthe BRAN specification has been described. The behaviors in thetransmission are specified assuming that the transmission destination ofan asynchronous packet exists. Behaviors which should be taken if thetransmission destination bus is disconnected for some reason are notconsidered.

Behaviors which should be taken in such a case will be specificallydescribed for a case shown in FIG. 16. A 1394 network 60 shown here isformed of a bridge 63 formed of a portal 61 connected to a bus BUS1 anda portal 62 connected to a bus BUS63, a bridge 66 formed of a portal 64connected to the bus BUS63 and a portal 65 connected to a bus BUS2, anode 67 connected to the bus BUS1, and a node 68 connected to the busBUS2.

For convenience of the description, an imaginary bus BUS3 which does notexist in the 1394 network 60 is defined, an imaginary node 69 connectedto the bus BUS3 is defined, and a case in which the node 67 sends apacket to the node 69 will be considered.

A packet sent from the node 67 is first received by the portal 61.Receiving the packet, the portal 61 determines whether the packet is tobe forwarded to an adjacent bus. In other words, the transmissiondestination bus ID of the packet is checked.

When the transmission destination bus ID is not the local-bus ID, thepacket is regarded as that bound for a node disposed on another bus, andis actually forwarded. To forward the packet, the portal 61 firstreturns an ACK indicating that the packet has been received, to the node67. Then, the portal 61, which received the packet, passes the packet tothe portal 62, positioned at the other side of the bridge 63, and theportal 62 sends the received packet to the bus BUS63 connected thereto.

For the packet sent from the portal 62 to the bus BUS63, however, aportal which is to receive the packet does not exist. A node connectedto a branch bus receives only a packet which is bound for the bus ID towhich the portal disposed at the other side of the bridge is connected.The corresponding portal does not exist. Therefore, the packet is notreceived on the bus BUS63.

An ACK is not returned to the portal 62, which transmitted the packet,and time-out occurs. In such a case, since the cause of the error cannotbe identified, meaningless re-transmission may be performed. This causesadditional traffic to occur on the bus, and is not preferred in terms ofstructuring the 1394 network 60.

In the present invention, a method for performing appropriate errorprocessing when the transmission destination bus of an asynchronouspacket does not exist is specified. An embodiment of the presentinvention will be described below by referring to FIG. 17.

A 1394 network 80 shown here is formed of a bridge 83 formed of a portal81 connected to a bus BUS1 and a portal 82 connected to a bus BUS63, abridge 86 formed of a portal 84 connected to the bus BUS63 and a portal85 connected to a bus BUS2, a node 87 connected to the bus BUS1, and anode 88 connected to the bus BUS2.

For convenience of the description, an imaginary bus BUS3 which does notexist in the 1394 network 80 is defined, an imaginary node 89 connectedto the bus BUS3 is defined, and a case in which the node 87 sends apacket to the node 89 will be considered.

A packet sent from the node 87 is first received by the portal 81.Receiving the packet, the portal 81 determines whether the packet is tobe forwarded to an adjacent bus. In other words, the transmissiondestination bus ID of the packet is checked.

When the transmission destination bus ID is not the local-bus ID, thepacket is regarded as that bound for a node disposed on another bus, andis actually forwarded. To forward the packet, the portal 81 firstreturns an ACK indicating that the packet has been received, to the node87.

The portal 81, which received the packet, passes the packet to theportal 82, positioned at the other side of the bridge 83, and the portal82 sends the received packet to the bus BUS63 connected thereto. For thepacket sent from the portal 82 to the bus BUS63, however, a portal whichis to receive the packet does not exist.

A node connected to a branch bus receives only a packet which is boundfor the bus ID to which the portal disposed at the other side of thebridge is connected. The corresponding portal does not exist. Therefore,a method for applying acknowledgement processing to the packet in theabove-described case is defined.

One of portals connected to the branch bus is selected. In thisselection, a portal having the largest, assigned, virtual ID may beselected. In the above case, the portal 82 and the portal 84 areconnected to the branch bus, and the portal 84 has a larger virtual ID.Therefore, the portal 84 is selected.

The portal 84 can obtain information indicating how many bridges areconnected to the branch bus. This information is obtained by checkingthe contents of self ID packets received when the bus is reset. A selfID packet which each node sends at a bus reset includes bits indicatingwhether the node itself is a bridge, as shown in FIG. 18A and FIG. 18B,and receiving of this packet enables the determination.

Since the self ID packet includes information related to the node ID,the node ID of each portal can be obtained as well as the number ofbridges connected to the branch bus. Each portal generates a tableindicating the correspondence between the node IDs and the virtual IDsfor nodes disposed on a bus, as shown in FIG. 19, and holds it.

From the information obtained as described above, the portal 84 canobtain in advance the number of bridges connected to the branch bus andthe bus IDs of connected buses. As shown in FIG. 20, when a flagindicating a portal is added to the table indicating the correspondencebetween the node IDs and the virtual IDs, the above information can bemanaged. According to this information, error processing is applied to apacket having a unclear destination.

The portal 84 usually forwards only asynchronous packets bound for thebus BUS2, which is the bus connected to the other side of the bridge.The following check is also applied to packets which the portal 84itself does not forward.

When the transmission destination bus ID of a received packet is checkedand it is determined that the packet is bound for a bus other than thebus BUS2, whether the transmission destination bus ID exists on the 1394network 80 is checked. This checking is performed by checking thevirtual IDs of portals connected to the branch bus.

In the case, the transmission destination bus ID is “3” whereas onlyportals having virtual IDs of “1” and “2” are connected to the branchbus. Therefore, it is found that the bus BUS3, which is the transmissiondestination, does not exist.

When the portal 84 understands that the packet is bound for thedestination which does not exist on the network, the portal 84 returnsack_address_err to the transmission source. When the transmission sourcenode receives this acknowledgement, it understands that the transmissiondestination does not exist. Therefore, re-transmission (retry) is notperformed. Error processing is thus performed. FIG. 21 shows theprocessing in a table.

FIG. 22 shows the structure of an interface apparatus which implementsthe bridges 83 and 86 shown in FIG. 17. As shown in FIG. 22, aninterface apparatus 100 which implements a bridge function is formed ofa central processing unit (CPU) 101, a memory 102, and portals 103A and103B all connected to a host bus HOSTBUS.

The portal 103A is formed of a link section 104A and a PHY section 105A.The link section 104A is formed of a direct memory access (DMA)controller section 106A, a first-in first-out (FIFO) section 107A, and alink core section 108A. In the same way, the portal 103B is formed of alink section 104B and a PHY section 105B. The link section 104B isformed of a DMA controller section 106B, a FIFO section 107B, and a linkcore section 108B.

The CPU 101 generates a table indicating the correspondence between nodeIDs and virtual IDs, as shown in FIG. 19 and FIG. 20, and makes the linkcore sections 108A and 108B hold the table. The link core sections 108Aand 108B refer to the table indicating the correspondence between thenode IDs and the virtual IDs to determine whether a received packet isto be forwarded. When the link core sections 108A and 108B determinethat the packet is not to be forwarded, they perform error processing.When it is determined that the packet is to be forwarded, the sectionsdirectly transmits and receives the packet.

The digital-serial-data interface apparatus 100 having a 1394-bridgefunction of the BRAN specification and having the foregoing structuredetermines whether the bus number indicated by the transmissiondestination address of an asynchronous packet transmitted on the busexists. When the destination bus does not exist, the interface apparatus100 returns error information to the transmission-source node.Therefore, the interface apparatus 100 can perform appropriateasynchronous transmission corresponding to the destination address of anasynchronous packet.

With the foregoing structure, when an asynchronous packet is sent to adestination which does not exist in a 1394 network, error informationindicating that the transmission destination address does not exist inthe 1394 network is sent to the transmission source. Re-transmission ofthe asynchronous packet is prevented, and thereby the frequency band ofthe network is efficiently used.

In the above-described embodiment, the portals 103A and 103B of theinterface apparatus 100 having a bridge function serve as data transferapparatuses. The present invention is not limited to this case. Thepresent invention can also be applied to other various data transferapparatuses which connect buses, and transmit data transferred throughits own bus to another bus, if necessary, according to destinationinformation attached to the data.

In the above-described embodiment, the link sections 104A and 104B serveas transmission means. The present invention is not limited to thiscase. The present invention can also be applied to other varioustransmission means which determine according to destination informationwhether the node to which data has been sent is connected to a bus, and,when it is determined that the node is not connected to the bus,transmit predetermined error information to the data transmissionsource.

In addition, in the above-described embodiment, the link sections 104Aand 104B serve as transmission means. The present invention is notlimited to this case. The present invention can also be applied to othervarious transmission means which determine according to destinationinformation whether the bus to which the node serving as the datatransmission destination is connected exists on a network, and when itis determined that the bus does not exit, transmit predetermined errorinformation to the data transmission source.

Furthermore, in the above-described embodiment, the link sections 104Aand 104B serve as transmission means. The present invention is notlimited to this case. The present invention can also be applied to othervarious transmission means which transmit data from the data transferapparatus to another data transfer apparatus according to destinationinformation.

In the above-described embodiment, the present invention is applied tothe portals 103A and 103B of the IEEE-1394 bridge conforming to the BRANspecification. The present invention is not limited to this case. Thepresent invention can be widely applied to data transfer apparatuseshaving other various specifications.

1. A data transfer apparatus for connecting a plurality of buses and fortransmitting data through a first bus to a second bus according todestination information related to the data, the data transfer apparatusbeing connected to the second bus through a second data transferapparatus, the data transfer apparatus comprising: transferring meansfor transferring the data from the data transfer apparatus to the seconddata transfer apparatus according to the destination information;determining means for determining, according to the destinationinformation, whether a node serving as a destination of the data isconnected to the second bus; wherein, when the determining meansdetermines that the node is not connected, a data transmission sourcereceives a predetermined error information signal, wherein thepredetermined error information signal prevents retransmission; whereinat least one packet of the data, transmitted to the node that is notconnected, is processed as an error packet; wherein the destinationinformation comprises a node ID and a bus ID; and wherein the datatransfer apparatus is formed of an IEEE-1394 bridge conforming to theBRAIN specification.
 2. The data transfer apparatus according to claim1, wherein the means for determining determines, according to thedestination information, whether the bus to which the node serving asthe destination of the data is connected exists on a network, andwherein when the bus does not exist, transmits predetermined errorinformation to the data transmission source.
 3. A network systemcomprising: a plurality of buses connected through a data transferapparatus, the data transfer apparatus for connecting the plurality ofbuses and for transmitting data transferred through its own bus to asecond bus according to destination information attached to the data,wherein the data transfer apparatus is connected to the second busthrough a second data transfer apparatus, and the data transferapparatus further comprises: transferring means for transferring thedata from the data transfer apparatus to the second data transferapparatus according to the destination information; determining meansfor determining, according to the destination information, whether anode serving as a destination of the data is connected to the second bus, and, when it determines that the node is not connected, a datatransmission source receives a predetermined error information, whereinthe predetermined error information signal prevents retransmission;wherein at least one packet of the data, transmitted to the node that isnot connected, is processed as an error packet; wherein the destinationinformation comprises a node ID and a bus ID; wherein the data transferapparatus is formed of an IEEE-1394 bridge conforming to the BRANspecification.
 4. The network system according to claim 3, wherein themeans for determining determines according to the destinationinformation whether the bus to which the node serving as the destinationof the data is connected exists on a network, and, when the bus does notexist, transmits predetermined error information to the datatransmission source.
 5. A data transfer method for a data transferapparatus for connecting buses and for transmitting data transferredthrough a first bus to a second bus according to destination informationattached to the data, the data transfer apparatus being connected to thesecond bus through a second data transfer apparatus, the data transfermethod comprising: a transferring step of transferring the data from thedata transfer apparatus to the second data transfer apparatus accordingto the destination information; a determining step of determining,according to the destination information, whether the node serving as adestination of the data is connected to one of the buses; and atransmitting step of, when it is determined that the node is notconnected, transmitting predetermined error information to a datatransmission source, wherein the predetermined error information signalprevents retransmission; wherein at least one packet of the data,transmitted to the node that is not connected, is processed as an errorpacket; wherein the destination information comprises a node ID and abus ID; wherein the data transfer apparatus is formed of an IEEE-1394bridge conforming to the BRAN specification.
 6. The data transfer methodaccording to claim 5, wherein, in the transmitting step, it isdetermined according to the destination information whether the bus towhich the node serving as the destination of the data is connectedexists on a network, and, when it is determined that the bus does notexist, predetermined error information is transmitted to the datatransmission source.